A form of intimidation or psychological abuse, sometimes called Ambient Abuse where false information is presented to the victim, making them doubt their own memory, perception and quite often, their sanity. The classic example of gaslighting is to switch something around on someone that you know they're sure to notice, but then deny knowing anything about it, and to explain that they "must be imagining things" when they challenge these changes.

A more psychological definition of gaslighting is "an increasing frequency of systematically withholding factual information from, and/or providing false information to, the victim - having the gradual effect of making them anxious, confused, and less able to trust their own memory and perception.

Your spouse begins telling you things that never really happened. For instance, he says that last week he told you he was going to go to the bar with his buddies this Monday night, but you never remember him telling you that.

Or maybe your boss gets angry because you didn't prepare the report he asked you to. When you remind him that he usually prepares that particular report, he snaps that he told you to take care of it a few days ago because he was too busy. However, you know he never asked you to do so.

Tommy Lee was gas lighting Elizabeth Taylor until she lost her mind. Mr. Lee should be shot.

Courtney Love leveled Kurt Cobain, not just with the multiple affairs, but by gently undermining his self respect with a little known procedure called gas lighting. He didn't even know he was a pound overweight until he was 20 pounds overweight. Poor guy.

It is a sign of the slow progress of culture that something as insidious as gas lighting can fail to be widely known, except in a slang dictionary definition, nearly three quarters of a century later than the movie by the same name.

Gaslighting is a systematic pattern of abuse by which the abuser manipulates factual information to give the victim the impression that they cannot trust their own senses. It should not be confused with discounting or minimizing, two very different forms of verbal abuse that are far more common.

When a ‘significant other’ or friend stops wanting to hang with you unless they can’t find anyone else to hang with.
Some believe this terminology roots back to the operation of a gas lantern and how it can be turned down in an instant as not to waste fuel (i.e. a persons time) and then turned back up once there is insufficient natural light (i.e. there’s no one else to hang with).

Unsuspecting Dupe: “Man, I don’t know what’s up with Emmy. Everything’s cool and then all of a sudden she won’t call for a couple weeks; then she’ll just start showing up again.”
Unsuspecting Dupes friend: “Yea, she’s gaslighting you bro’.”